Widespread use of DC defibrillators in patients suffering cardiac arrest has greatly increased the rate of successful resuscitation both in and out of hospitals over the past few decades. Defibrillation is applicable to life-threatening cardiac arrests resulting from ventricular fibrillation which occurs because of asynchronous depolarization of cardiac cells. When a sufficient electrical pulse is delivered to the heart from an external defibrillator through a set of paddles (electrodes), all cardiac cells briefly arrest and thereafter synchronous or normal depolarization may once again resume.
The defibrillator equipment presently offered to the medical arts discharges the electrical energy through an RLC circuit which is manually triggered by the physician, and the heretofore standard quantity of the electrical pulse to be delivered has been calibrated in terms of joules of energy. The many studies reported in the medical literature of attempts to determine the optimal electrical strength of the pulse that should be delivered for defibrillation are almost invariably analyzed in terms of joules. Delivery of more than enough electrical energy for defibrillation has been associated with cardiac cell death, yet insufficient energy will not accomplish the desired defibrillation, resulting then in multiple attempts to defibrillate at ever higher energy levels.
Previous recommendations for the "first attempt" defibrillation usually have been based on gross energy levels e.g., 200 joules. In fact, according to the STANDARDS AND GUIDELINES FOR CARDIOPULMONARY RESUSCITATION (CPR) AND EMERGENCY CARDIAC CARE (ECC)--published in JAMA, Vol. 225, pp 2942-2943, 1986, patients in ventricular fibrillation should receive DC countershocks of 200 joules (first shock), 200 joules (second shock), and 360 joules (third shock), as needed.
Selection of energy dose level for threshold defibrillation is believed to be sub-optimal for several reasons. For a given pulse duration, peak current is a better predictor of the defibrillation threshold than delivered energy. Lerman et al., "Relationship between Canine Transthoracic Impedance and Defibrillation Threshold: Evidence for Current-based Defibrillation." Journal of Clinical Investigation, Vol. 80, pp. 797-803, Sept., 1987). Establishment of the defibrillation pulse on the basis of total electrical energy, as has been done by prior workers in the art, does not apply a consistent level of peak current (amperage) because, in humans as well as dogs, transthoracic resistance varies within large ranges from one subject to the next. The implication of these findings is that defibrillation doses should be calibrated in units of (or at least based upon) current instead of energy.
An object of this invention is to provide a method and apparatus for automatically providing a preselected threshold level of peak current adequate for defibrillation, wherein the defibrillator capacitor is charged according to the transthoracic resistance of each patient in order to provide the selected peak current.
Additionally, it is an object of the invention to prospectively determine such transthoracic resistance automatically and prior to defibrillation by applying to the patient a low amplitude exploration current via the defibrillator electrodes.
Further objects of the invention and the advantages thereof will become apparent from the description which follows.